This invention relates to a method for applying a protective coating composition onto a polycarbonate substrate and to articles obtained therefrom. More particularly, it relates to the initial treatment of a polycarbonate substrate with a hydrolyzed silylated organic UV screen, followed by the application of a protective abrasion-resistant coating thereon.
Recently, the substitution of glass glazing with transparent materials which do not shatter or are more resistant to shattering than glass, has become widespread. For example, transparent glazing made from synthetic organic polymers is now utilized in public transportation vehicles, such as trains, buses, taxis and airplanes. Lenses, such as for eyeglasses and other optical instruments, as well as glazing for large buildings, also employ shatter-resistant transparent plastics. The lighter weight of these plastics in comparison to glass is a further advantage, especially in the transportation industry where the weight of the vehicle is a major factor in its fuel economy.
While transparent plastics provide the major advantage of being more resistant to shattering than glass, a serious drawback lies in the ease with which these plastics mar and scratch, due to everyday contact with abrasives, such as dust, cleaning equipment and ordinary weathering, Continuous scratching and marring results in impaired visibility and poor aesthetics, and oftentimes requires replacement of the glazing or lens or the like.
One of the most promising and widely used transparent plastics for glazing is polycarbonate, such as that known as Lexan.RTM. polycarbonate, sold by General Electric Company. It is a tough material, having high impact strength, high heat deflection temperature, good dimensional stability, as well as being self-extinguishing, and is easily fabricated.
Attempts have been made to improve the abrasion resistance of transparent plastics. For example, scratch-resistant coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium, such as alcohol and water, are known. U.S. Pat. Nos. 3,708,225, 3,986,997 and 3,976,497, for example, describe such compositions.
Copending U.S. application entitled "Silicone Resin Coating Composition", by However A. Vaughn Ser. No. 964,910) discloses another abrasion-resistant coating composition. This coating composition has been found to be highly desirable as a protective finish for plastic, as well as metal or metallized substrates. A particularly significant area of application for this Vaughn coating is in the glazing and optical lens industry. Since these areas required a coating of high optical clarity, coatings which show flowmarks, dirtmarks, or other marks which may impair visibility are undesirable. It was further found, as shown by Frye, U.S. Pat. No. 4,277,287, assigned to the same assignee as the present invention that the addition of a small amount of a polysiloxane polyether copolymer to the coating compositions disclosed in the aforementioned Vaughn application eliminates the occurrence of the undesirable flowmarks and the like, as well as providing other improvements in the hard coating.
As shown in my copending application Ser. Nos. 154,623 and 154,624, improved weatherability of the resulting polycarbonate composites were achieved by application of the above silicone hardcoat compositions by incorporating silylated UV screen as defined hereinafter into the silicone hardcoat prior to its application onto the polycarbonate substrate.
The present invention is based on the discovery that improved weatherability of polycarbonate substrates having protective silicone hardcoats on at least a portion of the surface of such polycarbonate substrate can be achieved if silylated UV screens are initially applied directly onto the polycarbonate substrate as a hydrolysis product prior to the application of the silicone hardcoat composition. Prior to the present invention, the weathering barrier was limited to about 600-700 hours of accelerated weathering shown by a QUV device sold by the Q-Panel Company, of Cleveland, Ohio. It was found that by directly treating the polycarbonate substrate with silylated organic UV screen hydrolysis product, in accordance with the practice of the invention, followed by the application of the hardcoat composition, the accelerated QUV weathering barrier was enhanced to 1400 hours.